Disclosed is an electrochemical device. The electrochemical device includes: (a) a composite separator including a porous substrate, a first porous coating layer coated on one surface of the porous substrate, and a second porous coating layer coated on the other surface of the porous substrate; (b) an anode disposed to face the first porous coating layer; and (c) a cathode disposed to face the second porous coating layer. The first and second porous coating layers are each independently composed of a mixture including inorganic particles and a binder polymer. The first porous coating layer is thicker than the second porous coating layer. The electrochemical device has good thermal stability and improved cycle characteristics.